1. Field of the Invention
The present invention relates to radiography using PSL (photo-stimulable luminescence), and more particularly to cassettes used in such radiography.
2. Description of the Prior Art
In conventional radiography ("X-ray photography"), a plate is made by forming one or more silver halide emulsion layers on a flexible film base which is supported within a light-tight cassette. The interior of the cassette is coated with one or more X-ray sensitive luminescent layers. The cassette containing an unexposed X-ray film plate is loaded into an X-ray machine, and after exposure the cassette and exposed X-ray film plate are removed for development and fixing of the latent image produced. This is usually done automatically by feeding the cassette into a light-tight apparatus in which the cassette is opened, the exposed film plate is extracted and chemically processed and a new, unexposed film plate is loaded into the cassette which is then re-closed, the reloaded cassette and developed film plate being delivered to respective exit slots of the processing apparatus.
In the PSL system, a PSL X-ray plate has applied thereto a layer of a photostimulable luminescent material which comprises a phosphor, for example a europium-activated barium fluorohalide, and a binder. The phosphor has the characteristic that it can be energised to a metastable excited state by X-rays, and can then be stimulated by visible or infrared light to return to the ground state with the emission of visible light (of a different wavelength from the stimulating light). The excited state has a half-life of at least several hours or days in the absence of stimulating light. A PSL plate is potentially re-usable many times. The technique is described in an article by Sonoda et al. in Radiology, Volume 148 (September 1983), at pages 833 to 838, and it offers the potential advantages of better image resolution at lower X-ray dosages for the patient.
The phosphor is deposited as a layer on a flexible base which also requires enclosure in a light-tight cassette.
Current practice in PSL radiography is to pass the exposed PSL plate in its cassette to an automatic processing machine in which the PSL plate is removed from the cassette, scanned, exposed overall to light to return the PSL material to its ground state and then reloaded into a cassette for reuse. For scanning, the exposed PSL plate is transported past a laser, typically a helium-neon laser emitting at a wavelength of 633 nm, which scans line-wise across the plate in front of a light-guide comprising a bundle of optical fibers whose input ends are arranged in a line across the path of the plate close to the laser scanning line for the reception of light emitted, typically at wavelengths close to 400 nm, when the PSL material is stimulated by the laser. The light-guide is arranged to pass the emitted light to a photo-multiplier tube or other receptor. The result is a storable electronic raster image. The electronic image may be subjected to any desired computer image-enhancement techniques and it may be displayed on a video display unit, fed to a laser printer for the production of a plain paper copy, or used to control a laser arranged to expose a photographic film plate to produce an X-ray plate of conventional appearance.
The cassettes used in PSL radiography must have external dimensions which, are compatible with those of conventional X-ray photography cassettes so that the PSL cassettes can be exposed in the cassette holder of a conventional X-ray machine. This is not, of course, to imply that all radiography cassettes are of the same format: they are not, they vary in format from about A5 paper size suitable for wrist X-rays to about A2 for chest X-rays and even larger. In fact, the practice has developed of depositing the phosphor layer on a conventional X-ray film base and of exposing it in a conventional X-ray photography cassette which is modified only in that it does not contain any X-ray sensitive luminescent layer.
Thus currently used PSL cassettes have not been designed with the specific purpose of PSL radiography in mind, and they suffer from a number of disadvantages which will be explained later in this specification.